MKS-50 finally repaired

Where did I leave off? Having replaced the BBD drivers and the 4051 that de-multiplexes the control voltages for chorus rate and volume, the noise remained. Replacing the underrated and likely half dead bypass capacitors improved the noise that the BBDs were injecting into the rails, but still didn't solve the problem.

I was on the verge of putting the MKS-50 back on the broken pile when I nudged something near the 4051 and the noise stopped! Initially I expected to find a broken trace or cold solder joint. When I first acquired it, the synth did not respond to MIDI messages due to a number of broken traces. Thanks to all but one of the mounting screws being missing, the PCB had flexed and cracked. The underside is a mess of bodge wires as a result. I had also found and reworked a few suspect solder joints.

Poking around with a meter revealed nothing, but a closer look at the component side found this.

Look closely at the capacitor array. Something isn't quite right.

The corner isn't supposed to be chauffeured and the common pin is missing!

Could this be the cause of the noise problem? The capacitor array is used to sample and hold the control voltages coming out of the 4051. These control the overall volume VCA as well as the VCAs and VCFs for each voice and the chorus rate. Without the common ground pin I'm amazed there weren't more problems. Rather than providing sample/hold the damaged array would capacitively couple all of these control voltages, meaning the chorus CV was being modulated by and indeed modulating filter cutoff, etc.

As the synth is already quite a mess (cracked PCB, bodge wires, missing case lid...) I made a repair horrible hack.

Awful hack, but it works!

As a point of interest, the construction of the MKS-50 is awful. A single layer board with a lot of jumpers, ICs seated at odd angles and some very poor soldering. The case design also makes it difficult to remove the PCB without flexing it. Roland gear tends to be fairly well made, so I'm not sure what happened here.

The MKS-50 lives and sounds great! Next up will be another horrible hack involving a sheet of aluminium and some extrusions to fashion a top for the case.

Juno 106 Resurrection

In my previous post, I mentioned that the 80017A VCA/VCF ICs in the MKS-30 will die at some point. I am expecting this because about two years ago, two of the six in my Juno 106 failed in the space of a few weeks. The 80017A is not actually an IC, it is in fact a "hybrid module" - a collection of SMD ICs and capacitors and printed resistors on a small ceramic PCB, which is then coated in epoxy. It is also notoriously unreliable. The theory goes that either some reaction occurs in the epoxy, causing it to become slightly conductive, or moisture becomes trapped inside it. One solution that many people have had success with is to remove the coating with a solvent. This seems to restore some modules to full health, but not all. Some also require rework due to poor soldering.

After my Juno expired, I removed all six 80017As, intending to give them a bath in Acetone and soldered sockets in their place to make future replacement easier. One of them had already been replaced by a previous owner and the solder pads had suffered. Then I got a Jupiter 6 and basket case Emulator II and fixing the Juno slipped down my list of projects.

Original 80017As. Two are dead and the others are on borrowed time.

On to late 2014 and having spent the last few months on a big project (to be revealed as soon as I get around to writing a post) and repairing the Emulator II (well almost, there are still some issues), I finally decided to tackle the Juno. I couldn't be bothered spending several days stripping the 80017As with no guarantee of success, so I opted for the alternative - the D80017A clones from Analogue Rennascince.

Bring on the clones!

Lots of passives.

One of the many calibration procedures.

Clones installed. Probe on one of 12 test points.

There isn't much more to write really. The clones arrived on a scored PCB panel and installation was simply a case of separating them and plugging them in to the sockets I installed earlier. An hour with a scope had everything calibrated according to Roland's original service manual and I finally have a working 106 again. Many people say the Junos are overrated as they are fairly limited, simple synths. This is true, but they do sound good.

Roland MKS-30 modification

I intend on getting a Kiwitechnics patch editor at some point, but the latest revision of it no longer supports emulation of the Roland PG-200 programmer, so cannot control a stock MKS-30. I believe the MKS-30 was the first MIDI module produced and it's MIDI implementation is severely limited - basically note on/off, no sysex or CC. The only way to backup patches is with an expensive and rare memory cartridge. Editing via the front panel is a painful process and the original PG-200 programmers go for as much as the synth itself, but thankfully Johannes Hausensteiner came up with a rewritten firmware and minor hardware modification that allows saving and loading patches over MIDI as well as realtime control. There is also a mod to control the speed of the chorus at the expense of being able to adjust the high pass filter, but I decided to forgo this.

Lid removed to expose the guts of the MKS-30. The mod comprises a replacement EPROM and a single wire from pin 12 of IC 40 on the main board to R12 on the chorus/PSU board.

Closeup of the main board changes. Rather than solder directly to the IC pin, I used a Molex connector so that the boards can be separated easily when I inevitably have to replace the VCF/VCA ICs. There are two conveniently positioned vias here with the right pitch for a 2.54mm header.

The other end of the wire. R12 is disconnected from the MIDI in circuitry and the free end connected to the wire, gunked down with silicon to stop it wandering. This turns the MIDI thru port into a MIDI out.

The infamous Roland 80017A filter/amp hybrids. Two of them have failed in my Juno 106 and it is only a matter of time before these fail too.

Replacement battery installed last week.

In other news, the intermittent front panel button (MIDI channel) is now behaving itself after some massaging. I haven't taken the panel board out to have a look, but I now believe these to be the same as the tact switches in the Jupiter 6, i.e. totally unobtainable.

Repairs

Long time no post! I have been meaning to post something for a while, but I didn't realise that it had been quite that long.

So what have I been up to? I have built a MIDIbox Seq V4 and MIDIbox CV. Neither are completely finished, but they are usable. I have also acquired some more retro toys in varying states of (dis)repair....

Only one of these actually works, so lots to do. Oh and my Juno 106 is now suffering from the dreaded 80017A VCA/VCF plague. I have removed these and will be giving them the acetone bath treatment in due course.

x0xb0x.

I built this in a few days, whilst waiting to acquire some rare transistors to finish the 9090. The 9090 took weeks.

The x0xb0x is a clone of the infamous Roland TB-303 "Transistor Bassline", designed by Adafruit Industries and originally supplied by them. Kits are currently available from several sources. I got mine from http://www.x0xsh0p.de.

The underside of the main PCB. This is split into two sections - towards the front is the synthesiser, using mainly original '80s "rare parts" and at the back is the sequencer. The ATmega for the latter is missing in this shot.

The top of the same PCB, with buttons and LEDs for the sequencer in the foreground and potentiometers for the synth at the back.

The main PCB mounted in the case, and wired to the IO board which contains the MIDI, DINSYNC and USB interfaces, CV and Gate out and the audio jacks. The power supply is also on this board.

The completed x0xb0x.

9090

A few photos and a brief (and bad) video of my 9090 drum machine. This is a clone of the Roland TR-909 voices, without the sequencer, designed by Trevor Page. Yes it is attached to a piece of cardboard, but this is temporary! For more information on the 9090 see: http://www.introspectiv.org/

I intend to build a sequencer, based on the MidiBox Seq V4. For more on this visit: http://www.ucapps.de/ (MidiBox) and http://www.synthage.com/ (the sequencer control surface).

Board 1 part completed. This contains the power supply, noise generator, bass, snare, low tom and mid tom drums.

Completed board 1

Completed board 2 comprising high tom, rim shot, hand clap, hi-hats and cymbals, plus the MIDI interface. The three large ICs are EPROMs containing the samples for crash and ride cymbals and hi-hats. All other voices are analogue.

The rather temporary card panel with potentiometers attached.

Close up of the EPROMs on board 2, also showing some of the rare parts - three types of long discontinued Japanese transistors.

The 9090 in action.